1. Field of the Invention
The present disclosure relates to an automatic transmission.
2. Discussion of the Background
An automatic transmission capable of providing 8-forward-speed transmission using a first planetary gear set for input, second and third planetary gear sets for varying the speed, and six engagement mechanisms has been developed (refer to, for example, Japanese Unexamined Patent Application Publication No. 2005-273768).
According to Japanese Unexamined Patent Application Publication No. 2005-273768, the first planetary gear set for input is formed from a first sun gear, a first ring gear, and a first carrier that supports a pair of first pinions meshed with each other, one of which is meshed with the first sun gear and the other is meshed with the first ring gear. The first pinions can rotate about their own axes and revolve around the first sun gear. That is, the first planetary gear set is a so-called double-pinion planetary gear set. When the carrier is fixed and if the sun gear is rotated, the ring gear rotates in the same direction as the sun gear. Accordingly, such a planetary gear set is also referred to as a “plus planetary gear set” or a “positive planetary gear set”. Note that when the ring gear is fixed and if the sun gear is rotated, the ring gear rotates in opposite direction to the sun gear rotation.
In the first planetary gear set, the first sun gear serves as a fixed element that is fixed to a transmission case, the first carrier serves as an input element that is connected to the input shaft, and the first ring gear serves as an output element. The first planetary gear set reduces the rotational speed of the first carrier serving as the input element and outputs the rotation from the first ring gear serving as the output element.
In addition, each of the second and third planetary gear sets for changing transmission gear is formed from a Ravigneaux planetary gear set including a second sun gear, a third sun gear, a second ring gear integrated into a third ring gear, and a second carrier that supports a pair of second pinions meshed with each other, one of which is meshed with the second sun gear and the second ring gear and the other is meshed with the third sun gear. The second pinions can rotate about their own axes and revolve around the second sun gear.
The Ravigneaux planetary gear set includes four rotary elements (i.e., a first rotary element, a second rotary element, a third rotary element, and a fourth rotary element). The first rotary element, the second rotary element, the third rotary element, and the fourth rotary element are arranged in sequence at intervals corresponding to the gear ratios in a collinear diagram (a diagram indicating the ratio of the relative speed of one rotary element to the speed of another rotary element in the form of a straight line) of the Ravigneaux planetary gear set. Then, the first rotary element corresponds to the second sun gear, the second rotary element corresponds to the second carrier integrated into the third carrier, the third rotary element corresponds to the second ring gear integrated into the third ring gear, and the fourth rotary element corresponds to the third sun gear.
The six engagement mechanisms are formed from a first wet multiple disc clutch that releasably connects the first ring gear formed from the output element of the first planetary gear set to the fourth rotary element formed from the third sun gear, a second wet multiple disc clutch that releasably connects the input shaft to the second rotary element formed from the second carrier, a third wet multiple disc clutch that releasably connects the output element formed from the first ring gear to the first rotary element formed from the second sun gear, a fourth wet multiple disc clutch that releasably connects the input element formed from the first carrier to the first rotary element formed from the second sun gear, a first brake that releasably fixes the first rotary element formed from the second sun gear to the transmission case, and a second brake that releasably fixes the second rotary element formed from the second carrier to the transmission case.
According to such a configuration, by setting the first wet multiple disc clutch in a connection mode and setting the second brake in a fixed mode, a first transmission gear ratio is achieved. In addition, by setting the first wet multiple disc clutch in a connection mode and setting the first brake in a fixed mode, a second transmission gear ratio is achieved. By connecting the first wet multiple disc clutch to the third wet multiple disc clutch, a third transmission gear ratio is established. By connecting the first wet multiple disc clutch to the fourth wet multiple disc clutch, a fourth transmission gear ratio is established.
Furthermore, by connecting the first wet multiple disc clutch to the second wet multiple disc clutch, a fifth transmission gear ratio is established. By connecting the second wet multiple disc clutch to the fourth wet multiple disc clutch, a sixth transmission gear ratio is established. By connecting the second wet multiple disc clutch to the third wet multiple disc clutch, a seventh transmission gear ratio is established. By setting the second wet multiple disc clutch in a connection mode and setting the first brake in a fixed mode, an eighth transmission gear ratio is established.
In addition, the above-described existing automatic transmission includes eight trains along the axis line of the input shaft. More specifically, from a side of a torque converter, a first train is formed from the fourth wet multiple disc clutch and the first brake, a second train is formed from the first planetary gear set, a third train is formed from the first wet multiple disc clutch, a fourth train is formed from the third wet multiple disc clutch (note that although the third wet multiple disc clutch seems to be in the same train as the first planetary gear set in a skeleton diagram, a piston and a hydraulic pathway for the third clutch are formed between the first wet multiple disc clutch and the output gear, in reality), a fifth train is formed from the output gear, a sixth train is formed from the second planetary gear set, a seventh train is formed from the third planetary gear set, and an eighth train is formed from the second wet multiple disc clutch and the second brake.